A cathode ray tube (CRT) display device has been used for displaying output data or images from, among other things, a TV receiver or a personal computer. The CRT display device includes a horizontal deflection circuit for deflecting an electron beam in a horizontal direction. The TV receiver or the personal computer (PC) supplies the horizontal synchronizing pulses to a horizontal deflection control circuit which supplies driving pulses to the horizontal deflection circuit. The horizontal deflection circuit supplies a sawtooth waveform to a horizontal deflection coil for deflecting the beam in the horizontal direction. The beam is deflected from a start position to an end position in one direction for displaying the image during a tracing period, and is deflected from the end position to the start position of the next scan line in an opposite direction during a re-tracing period. A pulse during the re-tracing period is supplied to a flyback transformer circuit, which generates at a secondary winding a high voltage, such as 25K volts, and supplies it to an anode of the CRT for accelerating the beam.
A smoothing capacitor is connected in parallel to the secondary winding and is charged by the high voltage during the re-tracing period. The charge stored in the smoothing capacitor is discharged through the anode circuit which forms a discharging circuit during the tracing period. A horizontal width of display area varies if the high voltage becomes unstable, and as a result, it is desirable to regulate or stabilize the high voltage on the smoothing capacitor.
Various efforts have been made to regulate the high voltage on the smoothing capacitor. Japanese patent application No. 63-261771 (Published Unexamined Patent Application No. 02-108373) discloses that an output voltage of an auxiliary transformer which is magnetically decoupled from the flyback transformer is added to a high voltage of the flyback transformer during the re-tracing period to compensate the variation of the voltage. Although the decoupling of the auxiliary transformer reduces a variation of the horizontal deflection waveform at the primary winding of the flyback transformer caused by the added compensating voltage, the horizontal deflection waveform is actually varied since the addition of the auxiliary voltage to the voltage of the secondary winding of the flyback transformer is transmitted to the primary winding of the flyback transformer. A small variation, such as 4%, of the horizontal deflection waveform could be allowed in the CRT display device of the current TV system, which has a ratio of 4 (horizontal) to 3 (vertical). However, such a small variation (4%) could not be allowed in the CRT display device of a high definition TV system, which has a ratio of 16 (horizontal) to 9 (vertical), or a high resolution CRT monitor of 1000.times.1000 pels, since the small variation becomes noticeable in the latter case.